Add one to the entourage. A newly discovered asteroid called 2014 OL339 is the latest quasi-satellite of Earth – a space rock that orbits the sun but is close enough to Earth to look like a companion.
The asteroid has been hanging out near Earth for about 775 years and it will move on about 165 years from now, say Carlos and Raul de la Fuente Marcos of Complutense University of Madrid in Spain, who have just described it.
Quasi-satellites orbit in resonance with Earth, allowing the planet’s gravity to shift the rock’s position much like an adult pushing a child on a swing, says Martin Connors, an astronomer at Athabasca University in Canada. The asteroid orbits the sun every 365 days, as Earth does, but Earth’s gravity guides it into an eccentric wobble, which causes the rock to appear to circle backward around the planet.
The asteroid, which is between 90 and 200 metres in diameter, is among several different categories of space rock in Earth’s retinue besides our one satellite, the moon. Rocks that hang out at a gravitational middle ground known as a Lagrange point, where they follow or lead Earth in its orbit, are called Trojans.
Mini-moons, meanwhile, are small asteroids that get sucked into Earth’s gravitational pull and orbit the planet, but only for a few months or a year, says Paul Chodas at NASA’s Near Earth Object Program. He spotted what appeared to be a mini-moon back in 2002, but it turned out to be the third rocket stage of the Apollo 12 lunar mission.
Most planets and even some large asteroids are accompanied by hangers-on. With four quasi-satellites catalogued so far, Earth comes in second only to Jupiter’s six, though the gas giant probably has many more that we can’t see. The same is probably true of Earth, as small space rocks are difficult to find – astronomers didn’t spot the first till 2004.
“If you go into your kitchen and you see some big cockroaches, you know there are a lot of little ones there, too,” Connors says.